What should I know about installing a subpanel?

big_jimJanuary 1, 2013

No electrical wiring FAQ would be complete without addressing the issue of subpanels. If, after reading the article below, you have any questions or feel that your situation is not covered by the article, please visit the Electrical Wiring Forum to ask any follow-up questions.

A very special thanks to Tom_O for writing this article and sharing it with us.

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This is a general guideline about the installation of a sub-panel in a remote building, hopefully it will contain the information you need. At the bottom will be a list of items you can omit if the sub-panel will be located in the same building as your service entrance. Most of the requirements would apply to any structure, but I have aimed this at installations on residential property.

If your installation is subject to inspection, you should talk to your local code enforcement official prior to starting work.

Wire - Type and size

You can use underground cable. The following capacities apply for copper cables:

Cable ampacities - 60C temperature rating

#10 - 30 amps

#8 - 40 amps

#6 - 55 amps (can be protected at 60 amps)

#4 - 70 amps

#2 - 95 amps (can be protected at 100 amps)

If you elect to use cable, it will need to be protected from damage if it emerges from the ground on the exterior side of a building. This protection can be rigid metal conduit, intermediate metal conduit or schedule 80 PVC. The protection must extend from the bottom of the trench to the height of 8' above grade, or where the cable enters the building, whichever is less.

The preferred method is to run PVC conduit with individual conductors. The minimum size should be 1 1/4", as this will allow you to run up to a 100 amp feeder. The following capacities are for copper conductors with THWN, THW or XHHW insulation:

Individual conductor ampacities - 75C temperature rating

#10 - 30 amps

#8 - 50 amps

#6 - 65 amps (can be protected at 70 amps)

#4 - 85 amps (can be protected at 90 amps for outbuildings. If this sub-panel is in a dwelling #4 may be good for 100 amps - see notes below)

#3 - 100 amps

Please see the notes at the end regarding wire ampacities and temperatures!

Number of conductors needed

For various reasons, it is best to run 4 conductors. This would be two 'hots', one grounded conductor (neutral) and one equipment ground (bare or green).

Depth of trench

For cable, you will need 24" of cover. For PVC conduit, you will need 18" of cover.

While you have your trench open, you should put in at least one spare conduit (3/4" minimum size) for future use. It would be a shame to dig up your yard again just to run a phone line.

Sub-panel main breaker

At your remote building, your sub-panel must have a main breaker if there will be more than six breakers used in your panel. It is recommended to put in a main breaker even if only a couple of circuits will be used initially. If the main breaker is to be back-fed, it must...

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